Organic light emitting diodes (OLEDs) typically comprise several layers of organic material sandwiched between conductive thin film electrodes, at least one of the organic layers being electroluminescent. When a voltage is applied to the electrodes, holes and electrons are injected from the anode and cathode, respectively. The holes and electrons migrate from the electrodes and through the layers of organic material. When a hole and an electron are in close proximity, they are attracted to each other due to an electrostatic Coulomb force. The hole and the electron may combine to form a bound state referred to as an exciton. An exciton may decay though a radiative recombination process or a non-radiative recombination process. Excitons decaying in an electroluminescent material may decay in a radiative recombination process to produce a photon.
A radiative recombination process can occur as fluorescence or as phosphorescence depending on the spin state of the electron and hole combination that formed the exciton. Specifically, the exciton formed by the combination of the hole and electron may be characterized as having a singlet or triplet spin state. Radiative decay of an exciton from a singlet state results in fluorescence, whereas radiative decay from a triplet state results in phosphorescence.
Approximately one quarter of the excitons formed in organic materials typically used in OLEDs are singlets with the remaining three quarters being triplets. Direct radiative decay from a triplet state to a singlet state is an inhibited or forbidden transition in quantum mechanics and, as such, the probability for radiative decay from a triplet state to a singlet state is generally very small. Unfortunately, the ground state of most organic materials used in OLEDs is a singlet state, which prevents radiative recombination of an exciton in the triplet state directly to a singlet ground state at ambient temperatures. Electroluminescence by fluorescence therefore typically dominates, resulting in a maximum quantum efficiency, defined as the efficiency of electrons and holes recombining to emit light, of about 25%.
Although direct radiative recombination from a triplet state occurs at an extremely slow rate in most organic materials, the recombination rate may be substantially increased by using species with a high spin-orbit coupling constant. For example, complexes of transition elements such as Ir(III) and Pt(III) have been employed in so-called phosphorescent OLEDs, as the high spin-orbit coupling constants of these species promote efficient radiative relaxation from a triplet state. As such, some or all of the approximately 75% of excitons in the triplet state may also transition efficiently to the singlet ground state and emit light.
Advantageously, these species also have a high intersystem crossing rate, allowing most of the singlet states to convert to triplet states and radiatively recombine. As such, both singlet and triplet excitons undergo radiative decay, resulting in a maximum theoretical quantum efficiency of 100%. The practical advantages of enabling both singlet and triplet excitons to undergo radiative decay to the singlet ground state are substantial. For example, phosphorescent OLEDs have now been shown to have superior device efficiencies when compared to fluorescent OLEDs.
In most phosphorescent OLEDs, the emissive layer is comprised of a phosphorescent emitter doped in a host material. Doping a host material to form an emitter limits concentration quenching by isolating emitting sites from one another, and serves to optimize the balance of electrons and holes in the device.
Host materials are chosen, as a design rule, to have a triplet energy gap that is greater than that of the phosphorescent dopant to prevent back energy transfer to the host and to confine triplet excitons within the emissive layer. There has therefore been a strong focus over the last decade to develop host materials with high triplet energy gap for phosphorescent blue and white OLEDs, where triplet energy gaps larger than 2.8 eV are typically required. Experimental evidence over the last decade has confirmed this requirement for phosphorescent OLEDs of various emission colours, and for a large variety of host and phosphorescent emitter combinations.
High triplet energy gap host materials are particularly important in devices having more than one emissive layer. For example, in devices having a blue phosphorescent emitter as well as one or more phosphorescent emitters with a lower triplet energy gap, such as a green or red phosphorescent emitter, a host with a triplet energy gap greater than that of the blue emitter is required. An example of a device that combines, in the same host material, blue, green, and red phosphorescent emitters is a white OLED. White OLEDs use the principle of additive color mixing to produce a white light. Specifically, white OLEDs are designed to emit a combination of blue, green, and red light such that the emitted light stimulates the three types of color sensitive cone cells in a human eye equally and, as such, minimizes hue. For example, a white OLED may be designed to mimic the white color of sun's emission spectrum. As explained above, it is commonly understood that high efficiencies may only be obtained when the triplet energy gap of the host material is higher than that of the phosphorescent emitter with the highest triplet energy gap, which, for a white OLED, would be the blue emitter.
Although hosts with high triplet energy gaps have been developed for use in blue and white phosphorescent OLEDs, these hosts are often lacking in other material parameters that affect device performance, including external quantum efficiency, drive voltage, and/or lifetime. As such, devices requiring a host material with a triplet energy gap greater than that of a blue phosphorescent dopant may suffer compared to devices requiring a host material of a relatively lower triplet energy gap.